This invention relates to the manufacturing of electric cables in general and more particularly to the SZ twisting of power cable conductors with a sector-shaped conductor cross section, taking into consideration particularly the mechanical forces occuring in this process.
For better space utilization of the cable cross section, it is customary to use cable conductors with sector-shaped conductor cross section in multiconductor power cables in the low and lower-medium voltage range (below 10 kV). The conductors are twisted together with a constant direction of twist to form the cable core. Sector conductors with and without a preliminary twist can be used. Sector conductors without a preliminary twist are twisted together without untwisting; strong torsional stresses then act on the sector conductors during the twisting process. Sector conductors with a preliminary twist are twisted together with back twist. In that case, the torsional stresses are relatively low ("Kabel-und Leitungsfertigung," Kombinat VEB Kablewerk Oberspree, VEB-Verlag Technik Berlin, 1976, page 200). In the twisting of sector conductors, it is customary to arrange forced guidance for each conductor ahead of the twisting process. Such forced guidance may consist of several profiled pulleys which are arranged in a straight line, one behind the other, and between which the respective conductors pass (DE-OS No. 22 11 111).
Besides the conventional twisting of the sector conductors with a uniform direction of twist, a type of twisting is also known, in which the sector conductors are twisted with a direction of twist which changes at intervals. Such a type of twisting, which is called "SZ twisting" has found acceptance in recent years in the manufacture of communication and power cables. However, the SZ twisting machines developed for this purpose cannot be used directly, without further development, for the twisting of sector conductors, since large mechanical forces must be controlled in the twisting of sector conductors because of the large conductor cross sections (more than 35 mm.sup.2).
In known apparatus for SZ twisting without preliminary twist, an oscillating lay plate arranged ahead of a twisting closer is coupled to the control guides for the sector conductors. With such a twisting device, only one or two twist lays per twist direction can naturally be generated (DE-OX No. 25 14 033). It has furthermore been previously proposed to twist the sector conductors which are straightened ahead of the twisting closer, by means of a twisting device arranged between a first and a second twisting closer. This device rotates with a direction of rotation which changes at intervals, so that the sector conductors are twisted for a first time in the first twisting closer and a second time in the same direction in the second twisting closer. The twisting device consists of one or two collet pulls in tandem. Optionally, non-rotating collet pulls can be arranged between the twisting points and the twisting device in order to always ensure the same distances between the controlling stationary and rotating parts of the twisting device, so as to obtain equal length of lay. The direction of rotation of the twisting device is changed at intervals which are matched in a manner known per se to the storage capacity of the apparatus. (DE-OS No. 27 42 662)
For SZ twisting round conductors with a larger conductor cross section, an SZ twisting machine is also known which contains two twisting tools, namely, an oscillating holeplate and a twisting head which rotates in an oscillating manner in the form of a caterpillar pull or a three-pulley device. While the hole plate is arranged in front of a first twisting closer, the twisting head is located between the first and a second twisting closer. (DE-OS No. 24 12 199).
There is further known a twisting method for the SZ twisting of base groups of communication cables, in which the elements to be twisted are fed to a twisting closer via a hole plate, are twisted immediately thereupon by means of an oscillating twisting head and are subsequently conducted unsupported in air, over a distance which is much longer than the distance of the reversal points of the twist direction in the twisted material. By application of a wrapping during the twisting process, the reversal points of the twist direction are prevented from untwisting while the material is freely conducted through the air (Wire and Wire Products, 1967, page 96). For such twisting purposes, twisting heads can also be used which are designed in the manner of a caterpillar pull (DE-OS No. 17 90 249) or consist of a deflection pulley which is arranged tangentially to the twisting axis and around which the material to be twisted is looped once (DE-AS No. 17 65 453).